Method and apparatus for biomass gasification

ABSTRACT

A system includes a gasifier configured to gasify a mixture of a biomass feedstock and an oxidant to generate a producer gas and a producer gas purification system configured to purify the producer gas from the gasifier. The system also includes a gas supply system configured to supply a gas that is not the oxidant to at least one of the gasifier, or the producer gas purification system, or any combination thereof.

BACKGROUND

The subject matter disclosed herein relates to gasification systems, and more particularly, to systems for biomass gasification.

Biomass may be gasified for use in the production of electricity, chemicals, synthetic fuels, or for a variety of other applications. Biomass gasification often involves the partial oxidation of biomass, resulting in production of combustible gases including carbon monoxide (CO), hydrogen (H₂), and traces of methane (CH₄), or in other words, producer gas. Producer gas can be used to run internal combustion engines, for example as a substitute for furnace oil, and can also be used to produce methanol, oxo-chemicals, and so forth. However, the production of producer gas by the biomass gasifier also generates ash, char, and other waste products due to incomplete gasification or impurities in the biomass. Additionally, the producer gas may be cooled and treated before being used. Cooling and treating consumes auxiliary power, which lowers the efficiency of the biomass gasification system. Therefore, a system that decreases the amount of power used in auxiliary functions may be desirable.

BRIEF DESCRIPTION

Certain embodiments commensurate in scope with the originally claimed invention are summarized below. These embodiments are not intended to limit the scope of the claimed invention, but rather these embodiments are intended only to provide a brief summary of possible forms of the invention. Indeed, the invention may encompass a variety of forms that may be similar to or different from the embodiments set forth below.

In a first embodiment, a system includes a gasifier configured to gasify a mixture of a biomass feedstock and an oxidant to generate a producer gas and a producer gas purification system configured to purify the producer gas from the gasifier. The system also includes a gas supply system configured to supply a recycle gas to at least one of the gasifier, or the producer gas purification system, or any combination thereof wherein the recycle gas comprises an exhaust gas, one or more gases separated from the exhaust gas, or a combination thereof, generated from a combustion system.

In a second embodiment, a system includes a gasifier configured to gasify a mixture of a biomass feedstock and an oxidant to generate a producer gas, a producer gas purification system configured to purify the producer gas from the gasifier and a combustion system configured to combust the producer gas from the producer gas purification system to generate an exhaust gas. The system also includes a pressure swing adsorption system configured to separate the exhaust gas from the combustion system into a first gas and a second gas, wherein the pressure swing absorption system is configured to supply the first gas to at least one of the gasifier, or the producer gas purification system, or any combination thereof.

In a third embodiment, a method includes gasifying a mixture of a biomass feedstock and an oxidant in a gasifier to generate a producer gas, purifying the producer gas from the gasifier using a producer gas purification system, combusting the producer gas to generate an exhaust gas using a combustion system, separating the exhaust gas into a first gas and a second gas using a pressure swing adsorption system, and supplying the first gas to at least one of the gasifier, or the producer gas purification system, or any combination thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic block diagram of an embodiment of a biomass gasification system with an exhaust manager and a gas supply system; and

FIG. 2 is a schematic block diagram of an embodiment of a biomass gasification system showing details of a biomass gasification reactor and producer gas scrubbers.

DETAILED DESCRIPTION

One or more specific embodiments of the present invention will be described below. In an effort to provide a concise description of these embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure.

When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are intended to be inclusive and mean that there may be additional elements other than the listed elements.

The disclosed embodiments include systems and methods for improved efficiency in a biomass gasification system. In particular, the disclosed embodiments include a gas supply system that may distribute gases (e.g., recycle gas, such as exhaust gas) to a biomass gasifier or producer gas purification system. In a typical biomass gasifier, the biomass is converted into a producer gas with various byproducts, such as char and ash accompanying the reaction. The gases may be added to the gas purification system, which lowers the temperature of the resultant producer gas and increases the total volume of the producer gas. Furthermore, the gas supply system may distribute gases, such as carbon dioxide, that are collected from the exhaust of the biomass gasification system. This reduces the amount of exhaust gases and fumes that would normally be expelled into the atmosphere.

FIG. 1 illustrates a biomass gasification system 10 that may recover carbon dioxide for use within a biomass gasifier 12. Within the biomass gasification system 10, a biomass feedstock 14 may be utilized as a source of energy to create producer gas, as described below. The biomass feedstock 14 may include alfalfa straw, bean straw, barley straw, coconut shell, coconut husks, corn cobs, corn fodder, cotton stalks, peach pits, peat, prune pits, rice hulls, safflower, sugarcane, walnut shell, wheat straw, wood blocks, wood chips, bagasse, or other biomass materials.

The biomass feedstock 14 may be introduced into the biomass gasification system 10 via a feedstock drying and processing system 16. The feedstock drying and processing system 16 may resize or reshape the biomass feedstock 14, for example, by chopping, milling, shredding, pulverizing, briquetting, or pelletizing the biomass feedstock 14. The resized or reshaped biomass feedstock 14 may be dried to create a dry feedstock 18. In certain embodiments, the feedstock drying and processing system 16 may include a grinding mill.

The dry feedstock 18 may be directed into the biomass gasifier 12. In particular, the dry feedstock 18 may be combined with a limited amount of oxidant 20 and subsequently partially oxidized. The oxidant 20 may include air, oxygen, oxygen-enriched air, or any combination thereof. By limiting the amount of oxygen (e.g., partial oxidation), however, and elevating/negative pressure (e.g., from absolute pressures of approximately 0.1 bar to 85 bar) and temperature (e.g., approximately 700° C. to 1600° C.), the dry feedstock 18 partially oxidizes and creates producer gas 22. Due to chemical reactions between the oxidant 20, possible addition of steam to provide heat, and carbon within the dry feedstock 18, the producer gas 22 may include hydrogen, carbon monoxide, carbon dioxide, water vapor, and methane, as well as other less desirable components, such as ash, dust, tar, and nitrogen.

From the biomass gasifier 12, the producer gas 22 is directed to a producer gas purification system 24. For example, the gas purification system 24 may include one or more scrubbers, water gas separators, gas separator solvent-based absorbers, flash tanks, filters, or any combination thereof. The producer gas purification system 24 removes some of the less desirable components from the producer gas 22, such as those listed above. To do so, the producer gas purification system 24 may use water and chilled water to purify the producer gas 22 into purified producer gas 26. The purification process is explained in connection with FIG. 2 below.

From the producer gas purification system 24, the purified producer gas 26 is directed to a power block 28 (e.g., a gas turbine or gas engine), which combusts the purified producer gas 26 to produce power 30, such as electrical power or mechanical power. The power 30 may then be used to operate other systems and/or the power 30 (e.g., electrical power) may be provided to a utility power grid. During combustion, the power block 28 produces engine exhaust 32, which may be at least partially recycled as a recycle gas for use elsewhere in the biomass gasification system 10. For example, the exhaust 32 (e.g., recycle gas) may be routed back into the gasifier 12, the purification system 24 or any combination thereof. By further example, the exhaust 32 (e.g., recycle gas) may be used to dry feedstock 14 in the feedstock drying and processing system 16.

Further, in certain embodiments, an exhaust manager 34 separates the engine exhaust 32 into several gases 36. The gases may include carbon dioxide or nitrogen gas. Separation of gases may be useful for environmental or economic reasons and may be performed by several methods, such as filtering, absorption, cryogenic distillation, or adsorption. In a particular embodiment, the exhaust manager 34 includes a pressure swing adsorption system 35, which separates carbon dioxide from the engine exhaust 32. The gases 36 may be transferred to a gas supply system 38 and deposited into several areas throughout the biomass gasification system 10. The gas supply system 38 may receive gases 36 from other than the exhaust manager 43 as well. The gas supply system 38 may operate in one of a plurality of modes, e.g., a first mode 40 or a second mode 42, for distributing the gases 36. In the first mode 40, the gas supply system 38 supplies the gases 36 to the biomass gasifier 12. In the second mode 42, the gas supply system 38 supplies the gases 36 to the producer gas purification system 24. Each of the first mode 40 and second mode 42 may operate independent of the other. Also, both modes may operate simultaneously. Each option is described in detail below in connection with FIG. 2.

With the foregoing in mind, FIG. 2 presents details of a biomass gasification process, particularly in distributing gases 36, such as carbon dioxide, to the biomass gasifier 12 and the producer gas purification system 24. The biomass gasifier 12 includes an inlet 44, a reactor 46, and an ash extraction system 50. The dry feedstock 18 and oxidant 20 enter the biomass gasifier 12 through the inlet 44. The dry feedstock 18 is mixed with the oxidant 20 in the reactor 46 of the biomass gasifier 12. Gravity assists to move the dry feedstock 18 towards the lower section of the reactor 46, as illustrated by arrow 52. As the dry feedstock 18 moves through the reactor 46, additional oxidant 20 may enter the reactor 46 through openings 54 and 56 located on the body of the reactor 46. The dry feedstock 18 is gasified within the reactor 46 to create producer gas 22.

Partial oxidation occurs in the biomass reactor 46. As part of the partial oxidation, the dry feedstock 18 may be heated to undergo a pyrolysis process. According to certain embodiments, temperatures inside the biomass reactor 46 may range from approximately 150° C. to 700° C. during the pyrolysis process, depending on the type of biomass feedstock 14 utilized to generate the dry feedstock 18. The heating of the dry feedstock 18 during the pyrolysis process may generate a solid (e.g., char) and residue gases (e.g., carbon monoxide, and hydrogen).

A combustion process may then occur in the biomass reactor 46. The combustion may include introducing an oxidant (e.g., pressurized or atmospheric air, or oxygen gas) to the char and residue gases. The char and residue gases may react with the oxidant to form carbon dioxide and carbon monoxide, which provides heat for the subsequent gasification reactions. According to certain embodiments, temperatures during the combustion process may range from approximately 700° C. to 1600° C. Next, steam may be introduced into the biomass reactor 46 during a gasification step. The char may react with the carbon dioxide to produce carbon monoxide and may react with steam to produce carbon monoxide and hydrogen at temperatures ranging from approximately 800° C. to 1100° C. The process by which the carbon dioxide combines with char to produce carbon monoxide is known as the Boudouard reaction. The Boudouard reaction is bidirectional, i.e. carbon monoxide may produce carbon dioxide and char (carbon) as well. Non-gasifiable char is undesirable, so it is beneficial for the Boudouard reaction to create carbon monoxide and reduce the amount of char. To this end, the gas supply system 38 may inject carbon dioxide obtained from the exhaust manager 34 into the biomass reactor 46. The injection of carbon dioxide may increase the amount of produced carbon monoxide by approximately 0.5 to 5, 0.75 to 3, or 1 to 1.5 percent. For example, the carbon monoxide production may increase from about 18 percent of the producer gas to about 19 to 19.5 percent of the producer gas. This increase in carbon monoxide also increases the calorific value of the producer gas by approximately 1 to 10, 1.5 to 5, or 2 to 4 percent and equivalently reduces the specific fuel consumption. In other embodiments, other gases 36 may also be injected into the biomass reactor 46 as well, increasing the overall volume of the producer gas 22.

Continuing with the description of the first mode 40, the amount of gas 36 injected into the biomass reactor 46, and the placement of the gases 36 may be finely controlled to optimize the gasification process. To control these and other functions, a controller 48 may electrically communicate with sensors throughout the biomass reactor 46. The controller 48 may sense and control temperature, flow rates, and notifications to system personnel. For example, if too much gas 36 is added to the biomass reactor 46, the controller 48 may sense that the temperature drops outside of the optimal range. The controller 46 may then adjust any of the parameters of the gas supply system 38 to overcome the detected problem. The gas supply system 38 may distribute the gases 36 in a number of places throughout the length of the biomass reactor 46. For example, the gases 36 may be added at the top of inlet 44. In some embodiments, inlet 44 is open to the atmosphere, allowing the reactor 46 to pull in oxidant 20 (e.g., air) and gases 36 automatically. Atmospheric addition into the biomass reactor 46 may reduce the need to pressurize the gases 36, and thus reduce the auxiliary power consumption by the gas supply system 38. On the other hand, pressurizing and forcing the gases 36 through openings 54 or 56 may increase efficiency of the entire system. The gases 36 may be forced through openings 54 and 56 with additional oxidant 20, or may replace the oxidant 20 that would have otherwise been added at openings 54 and/or 56. The biomass reactor 46 may also contain additional openings 58 devoted solely to addition of gases 36 from the gas supply system 38. Openings 58 may also be configured to prevent the producer gas 22 from channeling, that is, the openings 58 prevent the situation in which the producer gas 22 closest to the walls of the reactor 46 is stagnant and only the centermost producer gas 22 is flowing. Non-gasifiable ash material and unconverted and/or incompletely converted dry feedstock 18 may be byproducts of the process, and may be removed by the ash extraction system 50.

The producer gas 22 flows to inlet 44, as depicted by arrow 22, while the hot ash exits the reactor 46 via the ash extraction system 50. The ash extraction system 50 contains the hot ash until it is removed from the biomass gasifier 12. When the producer gas 22 enters the gasifier outlet 48, the producer gas 22 may be at a temperature in a range of approximately 300° C. to 500° C., 200° C. to 400° C., or 450° C. to 600° C., depending on the specific operating conditions of the biomass gasifier 12.

In certain embodiments, the gas purification system 24 may include first, second, and third scrubbers 62, 64, and 66, and other filters interconnected by tubes, pipes, or conduits. A separator 60, such as an inertial separator, gravity separator, or a cyclone separator may also be used in the biomass gasification system 10 to remove dust and other particles in the producer gas 22. For example, in certain embodiments, the cyclone separator 60 may be used to filter out particles greater than approximately 2, 3, 4, 5, 6, 7, 8, 9, or more micrometers. In certain embodiments, approximately 60 to 65 percent of the producer gas 22 may contain particles greater than 60 micrometers in size. Therefore, the separator 60 may remove a large number of particles from the producer gas 22. From the separator 60, the producer gas 22 flows through a tube 68 to the first scrubber 62. In the first scrubber 62, fines (e.g., fine particles), tar, and other entrained gases, such as hydrogen chloride, may be removed through processes such as water scrubbing, sour water stripping, absorption, decomposition, and/or selective stripping. In particular, within the first scrubber 62, the fines and tar may be separated from the producer gas 22 to produce a stream of scrubber water 70 that may exit a bottom portion of the first scrubber 62, while scrubbed producer gas 22 may exit through an upper portion of the first scrubber 62.

The scrubber water 70 exiting the bottom portion of the first scrubber 62 may be directed to a scrubber water processing system 72. The producer gas 22 exiting the upper portion of the first scrubber 62 flows through tube 74 to the second scrubber 64. In the second scrubber 64, additional fines, tar, and gases may be removed through processes similar to those used in the first scrubber 62. As with the first scrubber 62, the fines and tar may be separated from the producer gas 22 to produce a second stream of scrubber water 76 that may exit a bottom portion of the second scrubber 64, while scrubbed producer gas 22 may exit through an upper portion of the second scrubber 64. The second scrubber 64 may repeat the same scrubbing/purification process of the first scrubber 62, or a different method may be used in the second scrubber 64 to remove a different impurity from the producer gas 22.

The scrubber water 76 exiting the bottom portion of the second scrubber 64 may also be directed to the scrubber water processing system 72 and processed similar to the scrubber water 70 exiting the bottom portion of the first scrubber 62. The scrubber water processing system 72 also may include a settling process that produces separated fines and grey water and/or other byproducts. The separated fines may be recycled and used in the feedstock drying and processing system 16, where the fines may be used to provide additional fuel. The grey water and/or any other byproducts from the scrubber water processing system 72 may be directed to a water treatment unit 78 for further processing.

The producer gas 22 exiting the upper portion of the second scrubber 64 flows through a tube 80 to the third scrubber 66 (e.g., a chilled water scrubber). In the third scrubber 66, the producer gas 22 may undergo additional filtering and cooling. For example, chilled water may flow into the third scrubber 66 to exchange heat with the producer gas 22, thereby cooling the producer gas 22 and warming the water. Specifically, in certain embodiments, the water may flow through a first tube 82 to a chilled water tank 84, where the water is cooled for recirculation. The chilled water then flows through a second tube 86 to repeat the cycle of cooling the producer gas 22.

For a biomass gasification system 10 that produces megawatts of power, a large amount of energy may be used to cool the producer gas 22 in the third scrubber 66. To reduce this use of energy, the producer gas 22 may be cooled by adding gases 36 to one or more of the scrubbers 62, 64, or 66. In embodiments where the gases 36 are added, the amount of energy that the third scrubber 66 uses to cool the producer gas 22 may be reduced by approximately 0.5 to 10 or 1 to 5 percent, for example. In other embodiments, the first scrubber 62 may be entirely eliminated thereby further reducing the auxiliary power usage by approximately 1 to 5 percent.

In the second mode 42, the gas supply system 38 distributes gases 36 into one or more of the scrubbers 62, 64, or 66. Producer gas 22 in the scrubbers 62, 64, 66 may be over 200° C., as described above. Adding gases 36 that are much closer to room temperature reduces the temperature of the producer gas 22. As shown in FIG. 2, the gases 36 may be added to the first scrubber 62, the second scrubber 64, or the third scrubber 66, or any combination of the scrubbers. While the disclosed embodiment may still use the chilled water tank 84, the reduction in temperature may reduce the auxiliary power consumption the chilled water tank 84 uses to cool the water. In the example where the gases 36 include carbon dioxide, the addition of the gases 36 into the scrubbers 62, 64, and 66 may increase the mass flow rate to the gas engine 28 by approximately 1 to 15, 2 to 10, or 3 to 6 percent. The mass flow rate is the amount of producer gas 22 that is flowing to the gas engine 28. Thus, increasing the mass flow rate provides a related increase in the power produced by the gas engine 28. In certain embodiments, this increase in power produced by the increase in mass flow rate may be balanced somewhat by the lower calorific value of the producer gas 22 composition. In other words, the reaction occurring in the gas engine 28 has less reactants per volume of purified producer gas 26. The increase in mass flow rate, however, may increase the efficiency of the biomass gasification system 10 as a whole.

Similar to the first mode 40, the controller 48 includes sensors that monitor aspects of the gas purification system 24. The controller 48 may monitor the gases 36 being injected into scrubbers 62, 64, and 66 to ensure sufficient cooling of the producer gas 22. Additionally, the controller 48 may monitor the purified producer gas 26 to track the composition of the purified producer gas 26, thereby helping to improve combustion in the gas engine 28.

From the third scrubber 66, the producer gas 22 eventually flows through a tube 88 into the gas engine 28. In certain embodiments, other mechanical structures may assist with the transportation and cleaning of the producer gas 22, but eventually the producer gas 22 is combusted in the gas engine 28. The gas engine 28 may include a coal mine gas engine such as a Jenbacher engine, made by General Electric Company in Jenbacher, Austria. Such gas engines combust the purified producer gas 26 under a wide variety of conditions with high levels of durability and reliability. Gas engines range in power from approximately 0.25 to 10 MW and run on either natural gas or a variety of other gases (e.g., biogas, landfill gas, coal mine gas, sewage gas, combustible industrial waste gases). The Gas engine may be used in a broad range of commercial, industrial, and municipal facilities for on-site generation of power, heat, and cooling. Other engines may be used to combust the producer gas as well, including gas turbine engines or more simple combustion engines.

Technical effects of the invention include biomass gasification systems 10 configured to recover exhaust gases 32 from the biomass gasification combustion engine 28, and recirculate those gases within the biomass gasifier 12 and producer gas purification system 24. The systems described employ an exhaust manager 34 and a gas supply system 36 to receive and distribute exhaust gases 36 from the gas engine 28 within a biomass gasification system 10. The gas supply system 36 may passively or actively distribute gases within the biomass gasifier 12, thereby helping to improve the composition of the producer gas 22. In another example, the gas supply system 10 may distribute exhaust gases 36 into the producer gas purification system 24, thereby helping with cooling and assisting with purification of the producer gas 22.

This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims. 

1. A system, comprising: a gasifier configured to gasify a mixture of a biomass feedstock and an oxidant to generate a producer gas; a producer gas purification system configured to purify the producer gas from the gasifier; and a gas supply system configured to supply a recycle gas to at least one of the gasifier, or the producer gas purification system, or any combination thereof, wherein the recycle gas comprises an exhaust gas, one or more gases separated from the exhaust gas, or a combination thereof, generated from a combustion system.
 2. The system of claim 1, comprising a gas separator configured to separate the exhaust gas into at least a first gas and a second gas, wherein the gas supply system is configured to use the first gas as the recycle gas.
 3. The system of claim 2, wherein the first gas comprises carbon dioxide separated from the exhaust gas and used as the recycle gas.
 4. The system of claim 2, wherein the gas separator comprises a pressure swing absorption system.
 5. The system of claim 1, comprising the combustion system configured to generate the exhaust gas from a combustion of the producer gas.
 6. The system of claim 1, comprising a controller coupled to the gas supply system, wherein the controller has a first mode configured to provide the recycle gas to the gasifier and a second mode configured to provide the recycle gas to the producer gas purification system.
 7. The system of claim 1, wherein the gasifier comprises: a first opening configured to receive the oxidant; and a second opening configured to receive the recycle gas, wherein the second opening is separate from the first opening.
 8. The system of claim 1, wherein the gas supply system is configured to supply the recycle gas to the producer gas purification system at a lower temperature than the producer gas.
 9. The system of claim 8, wherein the producer gas purification system comprises one or more scrubbers, and the recycle gas is configured to cool producer gas in the one or more scrubbers.
 10. The system of claim 8, wherein the recycle gas comprises the exhaust gas, carbon dioxide separated from the exhaust gas, nitrogen separated from the exhaust gas, or any combination thereof.
 11. A system, comprising: a gasifier configured to gasify a mixture of a biomass feedstock and an oxidant to generate a producer gas; a producer gas purification system configured to purify the producer gas from the gasifier; a combustion system configured to combust the producer gas from the producer gas purification system to generate an exhaust gas; and a pressure swing adsorption system configured to separate the exhaust gas from the combustion system into a first gas and a second gas, wherein the pressure swing absorption system is configured to supply the first gas to at least one of the gasifier, or the producer gas purification system, or any combination thereof.
 12. The system of claim 11, wherein the producer gas purification system comprises at least one scrubber configured to scrub the producer gas from the gasifier, and the pressure swing absorption system is configured to supply the first gas to the at least one scrubber at a lower temperature than the producer gas to facilitate cooling.
 13. The system of claim 11, wherein the producer gas purification system comprises: a first water scrubber configured to receive the producer gas from the gasifier; a second water scrubber configured to receive the producer gas from the gasifier or the first water scrubber, wherein at least one of the first or second water scrubbers comprises a chilled water scrubber; a water source configured to supply water to at least one of the first or second scrubbers, wherein the first gas is configured to facilitate cooling in at least one of the first or second scrubbers.
 14. The system of claim 11, wherein the first gas comprises carbon dioxide.
 15. The system of claim 11, comprising a controller having a first mode and a second mode, wherein the first mode is configured to provide the first gas, the second gas, or the exhaust gas to the gasifier, wherein the second mode is configured to provide the first gas, the second gas, or the exhaust gas to the producer gas purification system.
 16. A method, comprising: gasifying a mixture of a biomass feedstock and an oxidant in a gasifier to generate a producer gas; purifying the producer gas from the gasifier using a producer gas purification system; combusting the producer gas to generate an exhaust gas using a combustion system; separating the exhaust gas into a first gas and a second gas using a pressure swing adsorption system; and supplying the first gas to at least one of the gasifier, or the producer gas purification system, or any combination thereof.
 17. The method of claim 16, wherein supplying the first gas comprises controlling a flow rate of the first gas based on a temperature of the producer gas in the gasifier.
 18. The method of claim 16, wherein the first gas comprises carbon dioxide.
 19. The method of claim 16, wherein supplying the first gas to the gasifier comprises facilitating a reaction to convert char into carbon monoxide.
 20. The method of claim 16, wherein supplying the first gas to the producer gas purification system comprises facilitating cooling in at least one scrubber used to scrub the producer gas. 